Wide range gaseous fuel combustion system for gas turbine engines

ABSTRACT

Systems capable of supplying either all gaseous fuel, all liquid fuel, or a mixture of the two, at a preselected ratio have been successfully employed in gas turbine engines. The mixing of such fuels has taken place in the combustion chamber and has caused the nozzle design and system controls to complicate the structure and increase costs. The subject system for burning gaseous fuels having a range of calorific values has overcome these problems by providing a simple inexpensive system. The system uses a starting step where only high calorific value gaseous fuel is injected through a first fuel supply passage of an injector and operates the engine to a preestablished engine operating parameter. After the preestablished engine operating parameter has been reached a low calorific value gaseous fuel is mixed in a mixing chamber outside the combustion chamber and injected through the first fuel supply passage until a level of heating value of the mixed fuel or a second engine operating parameter has been reached. The mixing continues and a second fuel supply passage of the injector is opened to increase the mass/volume flow of fuel into the engine. The ratio of low calorific value gaseous fuel increases and the high calorific value gaseous fuel decreases as the engine operating parameters are monitored until the engine operates on only the low calorific value gaseous fuel.

This is a division of Ser. No. 037,296, filed Apr. 9, 1987, now U.S.Pat. No. 4,761,948.

TECHNICAL FIELD

This invention relates generally to gas turbine engines and moreparticularly to a system for burning gaseous fuels having a range ofcalorific values.

BACKGROUND ART

Fuel systems capable of supplying either all gaseous fuel, all liquidfuel, or a mixture of the two at a preselected ratio have beensuccessfully employed in gas turbine engines. In some instances it isdesirable to be able to burn gaseous fuels having a range of calorificvalues in a gas turbine engine. It can be appreciated that in a placewhere a more economical fuel, such as a low calorific value fuel in agas producing area, is available, it would be more economical to arrangethe turbine engine to use only the low calorific value fuel to themaximum extent. In most gas producing areas the high calorific valuefuel or natural gas can be sold at a profit to a waiting customer,however; the low calorific value fuel is a by-product with little or nomarket value. Therefore, it is desirable to utilize such low calorificvalue fuel on site as a fuel. Such fuels, however, are difficult to usein a gas turbine engine. For example, it is very difficult to start agas turbine engine on low calorific value fuel. Furthermore, the lowcalorific value of the fuel will necessitate the burning of a greatermass/volume of fuel in order to achieve desired turbine inlettemperature within the turbine engine versus the mass/volume of fuelwhen burning high calorific value fuel.

A dual fuel system is disclosed, for instance in U.S. Pat. No.2,637,334, to N. E. Starkey issued May 5, 1953. The Starkey patentdiscloses a dual fuel system where liquid fuel is directed to a manifoldand then to a passage in the nozzle and a gaseous fuel is directed to aseparate manifold and then to a different passage in the nozzle.

U.S. Pat. No. 2,826,038 issued to James Forrest Shannon et al on Mar.11, 1958 discloses a gas turbine engine which utilizes a liquid fuelinjected into the combustion chamber by a fuel pump and a low calorificvalue gaseous fuel supplied through a separate duct into the combustionchamber. Shannon et al provides means for diverting a proportion of thepressure air from the combustion chamber and means for converting theenergy of the diverted air into useful work, whereby during the supplyof low calorific value fuel to the combustion chamber air may bediverted from the combustion chamber in proportion corresponding in massflow to the additional mass flow of fuel necessary to achieve thedesired nozzle temperature.

The primary problems encountered by the prior art systems are firstly,the different fuels have been injected into the combustion chamberthrough separate passages or ports each connected to the different fuelsource. The different calorific value of the fuel and correspondingdifferent mass flow rate required to provide necessary fuel quantitiesrequire different fuel injector designs. For example, orifice sizes mustbe different to supply the quantities of fuel needed to achieve properturbine inlet temperatures if proper fuel injector pressure drop, tocontrol combustion process, is to be maintained. If a fuel injector hasbeen designed for a high BTU fuel, unacceptably high fuel injectorpressure drops will occur at the fuel injector when running on low BTUfuel. On the other hand if a fuel injector has been designed to operateon low BTU fuel, very low fuel injector pressure drops will occur whenrunning on high BTU fuel; this could cause combustion drivenoscillations resulting in damage to the gas turbine. Secondly, the artteaches that the atomization and mixing of the different fuels withinthe combustion chamber to provide efficient burning (fuel to air ratioand mixing) and power requirements require controls to provide variedamounts of combustion air. Thirdly, starting a gas turbine engine on lowcalorific value fuel is very difficult to accomplish. Fourthly,complexity problems are encountered when trying to proportion thecombustion air needed to provide the appropriate fuel to air ratio forproper burning when burning the needed mass/volume flow of therelatively low calorific value fuel.

These problems as mentioned above complicate the structures, increasecost and complicate the system design used to burn fuels having a rangeof calorific values.

The present invention is directed to overcoming one or more of theproblems as set forth above.

DISCLOSURE OF THE INVENTION

In one aspect of the present invention a system for burning gaseousfuels having a range of calorific values in a combustion chamber isdisclosed. The system is comprised of at least one fuel injector locatedin the combustion chamber and having a first fuel supply passage and asecond fuel supply passage, a first throttle valve connected to thefirst fuel supply passage, a second throttle valve connected to thesecond fuel supply passage, a chamber connected to the first and secondthrottle valves, a first control valve connected to the chamber andbeing connectable to a source of high calorific value gaseous fuel, asecond control valve connected to the chamber and being connectable to asource of low calorific value gaseous fuel, and means for selectivelycontrolling the first and second throttle valves and the first andsecond control valves in response to preestablished engine parameter sothat one of the high calorific value gaseous fuel, the low calorificvalue gaseous fuel and a mixture of the high and low calorific valuegaseous fuels is selectively supplied to the gas turbine engine.

In another aspect of the present invention, a method of burning gaseousfuels having a range of calorific values in a combustion chamber of agas turbine engine comprises the steps of starting the engine byinjecting only a high calorific value gaseous fuel into the combustionchamber through a first fuel supply passage, monitoring an operatingparameter of the engine to determine when a first preestablishedoperating parameter of the engine has been established; mixing a lowcalorific value gaseous fuel with the high calorific value gaseous fuelin a mixing chamber when the preestablished parameter of the engine hasbeen established with the mixing step including the steps of decreasingthe quantity of high calorific value gaseous fuel and increasing thequantity of low calorific value gaseous fuel, controllably throttlingthe mixture of high and low calorific value gaseous fuel through thefirst fuel supply passage and a second fuel supply passage, and stoppingthe flow of high calorific value gaseous fuel and thereafter operatingthe engine on only the low calorific value gaseous fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the system used to burn gaseous fuelshaving a range of calorific values in the gas turbine engine;

FIG. 2 is an enlarged sectional view taken along lines II--II of FIG. 1;and

FIG. 3 is an enlarged sectional view taken along lines III--III of FIG.1.

BEST MODE FOR CARRYING OUT THE INVENTION

In reference to FIG. 1 a system 10 for burning gaseous fuels having arange of calorific values is shown in combination with a gas turbineengine 12. The engine 12 is of generally conventional design andincludes a compressor 14, a power turbine 16, an air supply system 18, acombustion chamber 20, and an ignition system 22.

The system 10 includes a fuel injector 26 located in the combustionchamber 20 and having a first fuel supply passage 28 centrally locatedtherein. The passage 28 has a plurality of outwardly directed orifices30, best shown in FIG. 3, each having a preestablished flow area openinginto the combustion chamber 20. The flow areaa of some of the orificesof the first passage are different than the preestablished flow area ofother orifices of the first passage to prevent hot spots on thecombustion chamber 20 walls. For example, the orifices are divided intoa first group of orifices 31 and a second group of orifices 32 having alarger flow area than the first group of the orifices 31. Morespecifically, when more than one fuel injector 26 is arranged in acircular pattern within the combustion chamber 20, the fuel injectedfrom adjacent fuel injectors into the combustion area in line with orbetween fuel injectors can cause hot spots. To insure against these hotspots, the second group 32 of the orifices 30 on each fuel injector 26is made smaller to reduce the fuel flow into the area in line with orbetween injectors where hot spots could be created. The orifices arearranged in an angular pattern selected to control the combustionprocess within the combustion chamber 20. The fuel injector 26 alsoincludes a second fuel supply passage 33 coaxially located with respectto the first passage 28. The second fuel supply passage has a pluralityof outwardly directed orifices 34, best shown in FIG. 2, each of whichhas a preestablished flow area opening into the combustion chamber 20.The orifices 34 also include a first group of orifices 35 and a secondgroup of orifices 36 having a larger flow area than the first group oforifice 35 to prevent hot spots from occurring as explained above. Inthe present application, the combined effective flow area of theorifices of the first fuel supply passage 28 and the combined effectivearea of the orifices of the second fuel supply passage 33 aresubstantially equal. In alternative applications, the effective area ofthe orifices in the first fuel supply passage 28 when compared to theeffective area of the orifices of the second fuel supply passage 33 maychange and may not be equal. A first throttle valve 37 is connected tothe first fuel supply passage 28 by a fuel line 38 and a second throttlevalve 40 is connected to the second fuel supply passage 33 by a fuelline 42. The first and second throttle valves 37,40 are connected to amixing chamber 44 by lines 46,48 shut-off valves 50,52 and lines 54,56,respectively.

The mixing chamber 44 is connected to a source of high calorific valuegaseous fuel 60 by a fuel line 62, a control valve 64 and a fuel line65. The mixing chamber 44 is also connected to a source of low calorificvalue gaseous fuel 66 by a fuel line 68, a control valve 70 and a fuelline 72. The control valves 64 and 70 are conventional electricallyoperated proportional valves in which the amount of opening is directlyproportional to the magnitude of the electrical signal applied thereto.

The system 10 further includes means 74 for selectively controlling thefirst and second throttle valves 37,40. The control means 74 includesmeans 76 for monitoring the engine 12 parameter and means 78 for varyingand monitoring the proportion of the high and low calorific valuegaseous fuels.

The control means 74 includes a sensor 79 which can be a speed or powersensor and an electronic control mechanism 90 of conventional designwhich receives input signals in a conventional manner. These signals canbe electrical, hydraulic or pneumatic and are converted into standardoutput signals for use by the control means 74. For example, the sensor79 is connected to the mechanism 90 with a line 92. The mechanism 90 isalso connected to the ignition system 22, the first throttle valve 37,second throttle valve 40, shut-off valves 50,52 and control valves 64,70by conventional feed lines 94,96,98,100,102,104,106, respectively.

Industrial Applicability

The system 10 is used to provide the gas turbine engine 12 with theability to burn gaseous fuels having a range of calorific values. Thegas turbine engine 12 is first started and brought up to rated speed ononly the high calorific value gaseous fuel. For example, the controlvalve 64 is opened to a maximum position, the shut-off valve 50 isopened to a maximum position and the throttle valve 37 is slightlyopened. High calorific value gaseous fuel flows from the source of highcalorific value gaseous fuel 60 through the connecting line 65, valve64, line 62, mixing chamber 44, line 54, shut-off valve 50, line 46,throttle valve 37, and line 38 into the first fuel passage 28 of theinjector 26, through orifices 30 and into the combustion chamber 20. Airfrom the air supply system 18 is mixed with the fuel, the ignitionsystem 22 is activated and the engine 12 is started and allowed toaccelerate in a conventional manner. Thereafter when the engine isoperational at a preestablished operating parameter, approximately25%-30% of the rated kW output power, the low calorific value gaseousfuel is allowed to mix with the high calorific value gaseous fuel byopening the control valve 70. More specifically in this embodiment asthe low calorific value gaseous fuel is mixed with the high calorificvalue gaseous fuel, the BTU content of the mixed fuel is reduced, thethrottle valve 37 opens further to pass the increased mass/volume offuel demanded by the engine 12 and the fuel pressure at the injector 26increases. Any further reduction in BTU content is similarly accompaniedby further opening of the throttle valve 37 and further increase in fuelpressure at the fuel injector 26. If this process were allowed tocontinue, the fuel pressure at the injector, due to the increasedmass/volume required by the decreasing BTU content of the fuel, willbecome unacceptably high. To avoid this, at a pre-established level ofBTU content of the mixed fuel as determined by relative position ofcontrol valves 64,70, before the fuel pressure at injector 26 becomesexcessive, control valve 52 is opened, thus allowing the mixed fuel toflow additionally through the second fuel supply passage 33. With theopening of the second fuel supply passage 33, there is a sudden increasein the mass/volume of fuel flow to the engine 12, the fuel pressure atthe injector inlet is considerably reduced and the throttle valves 37,40are repositioned towards the closed position to meet the engine fuelrequirements. Any further decrease in BTU content is handled by furthersimultaneous opening of the throttle valves 37,40, resulting inincreased fuel pressure at the injector inlet. Thus we see that theintroduction of the fuel into the second fuel supply passage 33 hasconsiderably widened the operational range of the engine on low BTUfuels. Simultaneous with the opening of the second fuel supply passage33, the minimum stops of throttle valves 37,40 are automaticallyadjusted in accordance with the BTU content of the mixed gas, to preventthe engine 12 from overspeeding in case of a sudden loss of load.

As an alternative to utilizing the relative position of the controlvalves 64,70, the relative position of the throttle valve 37 and asignal from the engine sensor 79 together with the ambient temperatureand pressure of the air entering the compressor 14 may be used as apreestablished parameter for opening the shut off valve 52 starting thesupply of fuel to the second fuel supply passage 33.

Thus the method of burning gaseous fuels having a range of calorificvalues in the combustion chamber 20 of the gas turbine engine 12comprises the steps of starting the engine 12 by injecting only the highcalorific value gaseous fuel through the fuel injector 26 into thecombustion chamber 20 through the first fuel supply passage 28,monitoring an operating parameter of the engine 12 to determine when thefirst preestablished operating parameter of the engine 12 has beenestablished; mixing the low calorific value gaseous fuel with the highcalorific value gaseous fuel in the mixing chamber 44 when thepreestablished operating parameter of the engine 12 is established withthe mixing step including the steps of decreasing the quantity of highcalorific value gaseous fuel and increasing the quantity of lowcalorific value gaseous fuel, controllably throttling the mixture ofhigh and low calorific value gaseous fuel through the first fuel supplypassage 26 and the second fuel supply passage 33, and stopping the flowof high calorific value gaseous fuel and thereafter operating the engineon only the low calorific value gaseous fuel. The mixing of the lowcalorific value gaseous fuel with the high calorific value gaseous fuelin the mixing chamber further includes monitoring the position of thecontrol valves 64,70 to determine when a preestablished heating value ofthe mixed fuel has been established. Additionally, supplying the fuel tothe second fuel supply passage 33 by opening the shut off valve 52prevents excessive fuel pressure at the fuel injector inlet.

With the invention as disclosed above, the system 10 for burning gaseousfuels having a range of calorific values has overcome the problem ofinjecting two different fuel through two separate passages or ports intothe combustion chamber by providing the mixing chamber 44 in which highand low calorific value gaseous fuels are mixed prior to injection intothe combustion chamber 20 through the first and second fuel supplypassage 28,33 in a single injector 26 into the combustion chamber 20.Additionally, excessive fuel pressure at the fuel injector inlet whenrunning on low BTU fuel has been avoided by use of the second fuelsupply passage 33. The simplicity of this system has provided low costand simple structural components. The problem of starting a gas turbineengine 12 when the main source of fuel is a low calorific gaseous fuelhas been eliminated by starting on only high calorific value gaseousfuel, mixing the high and low calorific value gaseous fuel in a mixingchamber 44 during an interim change-over phase and subsequentlyoperating the gas turbine engine on only low calorific value gaseousfuel after the mixing operation. The subject system 10 is capable ofburning two different calorific value gaseous fuels 60,66 in anyproportions by mixing the low and high calorific value fuels outside thecombustion chamber. Excessive fuel injector pressure drop or combustiondriven oscillations caused by insufficient fuel injector 26 pressuredrop has been overcome by providing two separate fuel passages 28,33 inthe fuel injector 26; passage 33 can be switched on or off, as required,to maintain proper fuel injector pressure drop.

Other aspects, objects and advantages of this invention can be obtainedfrom a study of the drawings, disclosure and the appended claims.

We claim:
 1. A method of burning gaseous fuels having a range ofcalorific values in a combustion chamber of a gas turbine engine,comprising the steps of:starting the engine by injecting only a highcalorific value gaseous fuel into the combustion chamber through a firstfuel supply passage; monitoring an engine operating parameter todetermine when a preestablished value of that operating parameter isachieved; mixing a low calorific value gaseous fuel with the highcalorific value gaseous fuel in a mixing chamber when the preestablishedvalue of that parameter is achieved, said mixing step including thesteps of decreasing the quantity of high calorific value gaseous fueland increasing the quantity of low calorific value gaseous fuel;controllably throttling the mixture of high and low calorific valuegaseous fuel through the first fuel supply passage and a second fuelsupply passage; stopping the flow of high calorific value gaseous fueland thereafter operating the engine on only the low calorific valuegaseous fuel.
 2. The method of claim 1, wherein said step of mixing thelow calorific value gaseous fuel with the high calorific value gaseousfuel includes the steps of:controllably opening a first control valve toincrease the flow of low calorific value gaseous fuel; controllablyclosing a second control valve to decrease the flow of high calorificvalue gaseous fuel.
 3. The method of claim 2, wherein said step ofmixing the low calorific value gaseous fuel with the high calorificvalue gaseous fuel further includes:monitoring the position of the firstand second control valves to determine an existing heating value of themixed fuel.
 4. The method of claim 1, wherein said step of controllablythrottling the mixture of high calorific value gaseous fuel and the lowcalorific value gaseous fuel through the first passage and the secondpassage includes the steps of:controllably opening a first and secondthrottle valve to increase the flow of mixed high and low calorificvalue gaseous fuel through the second fuel supply passage when a secondpreestablished value of that operating parameter is achieved.
 5. Themethod of claim 4, wherein said step of monitoring the engine operationparameter to determine when a preestablished value of that parameter isachieved includes an engine sensor, a control valve and a signaltherefrom and said step of controllably throttling the mixture of highcalorific value gaseous fuel and the low calorific value gaseous fuelthrough the first passage and the second passage includes the stepsof:controllably opening the throttle valves in response to the relativeposition of the control valve supplying the fuel to the first fuelsupply passage, the signal from the engine sensor and the temperatureand pressure of the air entering the compressor.
 6. The method of claim1 wherein said step of controllably throttling the mixture of highcalorific value gaseous fuel and the low calorific value gaseous fuelthrough the first passage and the second passage includes the stepsof:establishing the level of heating value of the mixed fuel;controllably opening a throttle valve to allow the flow of mixed highand low calorific value gaseous fuel through the second fuel supplypassage; and controllably closing a throttle valve to decrease the flowof mixed high and low calorific value gaseous fuel through the firstfuel supply passage.
 7. The method of claim 5 wherein said step ofcontrollably throttling the mixture of high calorific value gaseous fueland the low calorific value gaseous fuel through the first and secondsupply passage includes the steps of:controllably opening and closingthe throttle valves thereby simultaneously controlling the flow of mixedhigh and low calorific value gaseous fuel through the first and secondsupply passages to control the engine operation.
 8. The method of claim1 wherein said step of stopping the flow of high calorific value gaseousfuel includes the steps of:opening a control valve to a maximum positionallowing full flow of low calorific value gaseous fuel; and closing acontrol valve to a closed position preventing flow of high calorificvalue gaseous fuel.
 9. The method of claim 1 further including the stepsof:monitoring the magnitude of a signal going to a control valve used toincrease the flow of low calorific value gaseous fuel; and monitoringthe magnitude of a signal going to a control valve used to decrease theflow of high calorific value gaseous fuel.
 10. The method of claim 8wherein said step of stopping the flow of high calorific value gaseousfuel includes the steps of:monitoring the magnitude of the signal goingto the low calorific fuel control valve until the low calorific fuelcontrol valve is at a maximum open position; and monitoring themagnitude of the signal going to the high calorific fuel control valveuntil the high calorific fuel control valve is at its closed positionpreventing flow of high calorific value gaseous fuel.
 11. The method ofclaim 10, wherein said step of mixing the low calorific value gaseousfuel with the high calorific value gaseous fuel includes the stepsof:monitoring the magnitude of the signal going to the control valveused to increase the flow of low calorific value gaseous fuel;monitoring the magnitude of the signal going to the control valve usedto decrease the flow of high calorific value gaseous fuel; controllablyopening the control valve to increase the flow of low calorific valuegaseous fuel; and controllably closing the control valve to decrease theflow of high calorific value gaseous fuel.